I think it was placed in the Action poll to balance the candidates in each poll, for no other reason than that. There are many anime listed here that would fall under multiple categories, after all.
Virtually every anime, really all entertainment, could be described by multiple genres. Mahouka could also have fit the supernatural category well. As you said, I wanted categories to have similar number of entries to ensure fairness. Having one category with three titles and another with twelve would not have worked. Rather than having too many, my bigger concern was some categories having too few. That is why I skipped genre staples like romance and horror. There were not enough of each, all could comfortably be included in broader genres, like drama and supernatural. Science fiction would have been an uncomfortable fit for Mahouka. It's not incorrect. It takes place in the future, engineering and technology are important parts of the story - even more so in the novels. But that is not the series primary appeal. Action and magic are the driving forces behind the show. Either the action or supernatural genre worked best. At some point genre discussions are nothing more than botany. Whether Mahouka (or any other anime) is action, supernatural, or science fiction is irrelevant as long as it is included in an appropriate category, competing against similar anime, where fans have a chance at a fair vote. Science fiction probably would have been less fair, both to Mahouka and the other anime in the category. Competing against a list of mostly mecha and other shows that Mahouka has little in common with would not be fair, or equitable, to any of the series involved.
